Printing according to an electronic photographic system is well known in the art. For instance, a zero printing system disclosed by U.S. Pat. No. 2,576,047 and an electronic printing apparatus disclosed by Japanese patent application Publication No. 1554/1968 are well known in the art. In this prior art, a toner image is formed on a photo-conductive plate according to an electrostatic photographic method and is fixed, so that a non-photo-sensitive insulating pattern, namely, a printing master, is formed on the photo-conductive plate. In succession, the printing master is uniformly charged and is uniformly exposed to light, whereby the charges are held only on the image region which is not photo-sensitive, while the charges are removed from the photo-sensitive region. Therefore, by applying charged toner to the partially charged plate, a toner image is formed thereon. The toner image is then transferred onto a suitable image support. The above-described operation is repeatedly carried out to obtain prints.
More specifically, the aforementioned Japanese patent Application No. 1554/1968 discloses an apparatus as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a rotary drum which has pawls on the cylindrical wall thereof, to hold a photo-sensitive sheet 10. Provided around the drum 1 are a cleaning unit 8, a charging unit 4, a uniform exposure lamp 13, a developing unit 5, a transferring corotron 6 and an infrared fixing unit 7 for fixing a toner image on the photo-sensitive sheet 10.
The printing master is formed as follows: A photo-sensitive sheet 10, after being wound on the drum 1, is charged by the charging unit 4. The image of an original on a platen 3 which is illuminated by an illuminating lamp 14 is projected onto the photo-sensitive sheet 10 by a projecting lens 2, so that an electrostatic latent image is formed therein. The latent image is visualized by the developing unit 5, and the toner image is fixed on the photo-sensitive sheet 10 through fusion by the infrared fixing unit 7, to thus form the printing master.
The method of obtaining a number of copies from the printing master thus formed by electrostatic printing proceeds as follows: The printing master having the toner image on the photo-sensitive sheet is charged by the charging unit 4 and is then uniformly exposed to light by the uniform exposure lamp 13. The charges on the photo-sensitive sheet 10 charged uniformly are selectively caused to flow off by uniform exposure with the exception of those on the toner image region which is not photo-sensitive and not photo-conductive. Thus,a printing master having charges on only the toner image region is formed. The printing master thus formed is subjected to visualization by the developing unit 5. A transferring sheet 11 is delivered to the transferring section in synchronization with the rotation of the drum 1. The toner image on the printing master is transferred onto the transferring sheet by the transferring corotron 6 and then the image thus transferred is fixed by the fixing unit. Thereafter, the sheet is discharged into a sheet discharge tray, to thus form a copy of the printing master. The toner remaining on the printing master after printing is removed by the cleaning unit 8, to complete one printing cycle. Then, the printing cycle as described above is repeatedly carried out at many times as the required number of copies.
In the above-described method, an infrared fixing unit 7 is used to fix the toner image on the photo-sensitive sheet 10 in the course of forming the printing master. The present inventors have compared this method with other fixing methods by measuring potentials at image regions on the printing masters thereof. The comparative results are as indicated in Table 1 below:
TABLE 1 ______________________________________ Type of fixing unit Potential at image region ______________________________________ Infrared fixing unit -50 V Flash fixing unit -100 V Hot roll fixing unit -540 V Pressure roll fixing unit -500 V ______________________________________
As is apparent from Table 1, the decrease in the potential at the image region when fixing is carried out with the infrared fixing unit or the flash fixing unit is larger than that of the potential at the image region when fixing is carried out with the hot roll fixing unit or the pressure roll fixing unit. Thus, the hot roll fixing method or the pressure roll fixing method may be employed as a fixing method in the formation of a printing master, and can maintain a sufficiently high potential at the image region. However, in either the hot roll fixing method or the pressure roll fixing method, the following operation must be carried out: After being peeled from the drum 1, the photo-sensitive sheet on which the toner image has been formed is delivered through a guide to the fixing unit, to be subjected to fixing. The photo-sensitive sheet thus treated is then delivered back to the drum 1, so that it is again supported on the drum. That is, the hot roll fixing method and the pressure roll fixing method are not preferable in that they require intricate, expensive auxiliary techniques.